This research aims to establish the technique for heating the ions in a fast-flowing plasma efficiently and for converting the increased thermal energy into flow one by using a magnetic nozzle. Various technologies accumulated in plasma science and fusion researches help the development of the thruster with variable thrust, which is one of the representative in the advanced space transportation systems. We investigated in detail the ion heating method of a fast-flowing plasma and the energy conversion in a magnetic nozzle, and the following important results were obtained.1.Shear Alfven wave of m=-1 was excited efficiently with a helical-type antenna, and strong ion heating was observed in a fast-flowing plasma in the low density region.2.The effect of ion-ion collisions, the Doppler effect and the effect by the neutral gas were observed. These findings were useful for the efficient ion heating.3.We have observed experimentally that the thermal energy of ions heated nearly 100eV was converted to flow energy by passing through a divergent magnetic nozzle. It was demonstrated for the first time that the exhaust plasma flow energy could be controlled by changing an input RF power for the ion heating.4.It was also experimentally confirmed that an electric field was generated in the magnetic nozzle in the direction of the ion acceleration, and the ion velocity distribution function was measured. It was suggested that not only the energy conversion by the magnetic nozzle but also the effect of the electric field were important for the plasma acceleration.As a result, we have shown experimentally that the plasma acceleration could be controlled by the ion heating, which is a fundamental technology for the efficient space thruster, and clarified the plasma acceleration mechanism and its control methods. In this research, we have obtained a lot of important knowledge related to the establishment of the ion heating and the energy conversion technology.